For measuring the concentration of a particular component in a sample liquid, there exists a method in which the sample liquid containing an unknown concentration of the particular component and a reference liquid containing a known concentration of the particular component are bridged for measuring a potential difference generated therebetween to thereby compute the concentration of the particular component from the measured potential difference.
In this method, a concentration measuring plate is set, for example, to an analyzing apparatus for determining the concentration of the particular component.
The analyzing apparatus includes at least a set section for setting the plate, two probes, and computation means for computing the concentration of the particular component from the potential difference between the probes.
The plate includes at least a first liquid receiving portion to which a reference liquid is applied, a first terminal for conduction with the reference liquid applied to the first liquid receiving portion, a second liquid receiving portion to which a sample liquid is applied, a second terminal for conduction with the sample liquid applied to the second liquid receiving portion, and a bridge for shorting between the reference liquid in the first liquid receiving portion and the sample liquid in the second liquid receiving portion.
When the plate is set to the set section of the analyzing apparatus, the probes of the analyzing apparatus come into contact with the first terminal and the second terminal for measuring the potential difference between the reference liquid and the sample liquid. The computation means computes the concentration of the particular component of the sample liquid based on the measured potential difference.
A double pipette may be used for applying the reference liquid to the first liquid receiving portion and for applying the sample liquid to the second liquid receiving portion of the plate. For example, use maybe made of such a double pipette that includes a first pipette for drawing and dispensing the reference liquid through a liquid dispensing orifice and a second pipette for drawing and dispensing the sample liquid through a liquid dispensing orifice. The first pipette and the second pipette are held in a casing for example so that the distance between respective liquid dispensing orifices corresponds to the distance between the first liquid receiving portion and the second liquid receiving portion of the plate. The liquid dispensing orifice of each pipette may be an opening of a tip attached to the tip end of the pipette.
The double pipette may have the following problems when the reference liquid previously drawn in remains in the tip. Firstly, the amount of dispensing fluctuates when the remaining reference liquid is dispensed together with the reference liquid later drawn in. Such fluctuation of the dispensing amount also occurs when the first pipette cannot draw a liquid by a constant amount. Secondly, when the drawing and dispensing of a reference liquid is performed after a long interval from the previous drawing and dispensing of the reference liquid, the reference liquid remaining in the tip as adhered to the inner surface concentrates. The mixing of the concentrated liquid into the later sucked reference liquid causes the concentration of the reference liquid to vary.
When a sample liquid previously drawn in remains in the tip, the following problems may occur. Since the concentrations of a particular component may differ between the sample liquids to be measured, when the remaining sample liquid is mixed in a sample liquid to be measured next, the concentration of the sample liquid may vary.
Therefore, in the prior art double pipette, the tip of each pipette is replaced every time the reference liquid or the sample liquid is dispensed for avoiding the influence of the reference liquid or sample liquid previously drawn in and dispensed. However, this causes the following problems.
Firstly, since a large number of tips need be used, the pipette is disadvantageous in terms of the cost. The number of tips disposed of (as waste) is correspondingly large. Secondly, although the same reference liquid is used for plural measurements, the reference liquid need be drawn in each time of the measurements. Therefore, the drawing work is troublesome when a large number of sample liquids need be measured. Moreover, even when a tip is replaced every time of the drawing and dispensing of the liquid, the problem of the sucking amount fluctuation is not solved. Therefore, it is still impossible to dispense the liquid by a fixed amount.